The present invention relates to a light emitting device using a light emitting diode (LED).
FIG. 16 is a perspective view showing a conventional light emitting device disclosed in Japanese Patent 3,302,203. The light emitting device 1 comprises a substrate 2 made of resin such as epoxy resin, a pair of electrodes 2a and 2b fixed to the substrate 2 at upper and lower side thereof, an LED 3 mounted on the electrode 2a and connected to the electrode 2b by a wire 4, and an sealing member 5 made of a transparent resin for sealing the LED 3.
The light emitting device 1 is mounted on a print substrate 6, connecting the electrodes 2a and 2b with a pair of conductive patterns 6a and 6b. 
When current is applied to the LED 3 from the patterns 6a and 6b through the electrodes 2a and 2b, the LED 3 emits light 7.
The thickness of the light emitting device 1 can be reduced because the device is mounted on the print substrate 6 through thin electrodes 2a and 2b. However, the substrate 2 made of resin such as epoxy resin has a very low heat conductivity which is about one-hundredth of that of copper. Therefore, heat emitted from the LED 3 scarcely transmitted to the print substrate 6. Consequently when a high current is applied to the LED in order to produce high brightness, the emitted heat becomes high temperature, which causes junctions in the LED to degrade, and the sealing member 5 to discolor which decreases the transparency of the member. Thus, the quality of the LED device largely reduces.
FIG. 17 is a perspective view of another conventional light emitting device disclosed in Japanese Patent Laid Open 11-307820, which is improved in heat radiation.
The light emitting device 10 comprises a pair of conductive members 11a and 11b made of metal having high heat conductivity, an insulation member 12 made of resin for insulating the conductive members 11a and 11b and combining the members. The insulation member has an opening 13 having an elongated circular shape. A part of each of the conductive members 11a, 11b is exposed in the opening. An LED 14 is secured to exposed parts of the conductive members 11a, 11b, so that the LED 14 is electrically and thermally connected to conductive members 11a and 11b. The LED 14 is sealed by a transparent sealing member 15.
The light emitting device 10 is mounted on a print substrate 16, and the conductive members 11a and 11b are connected to a pair of conductive patterns 16a and 16b by solders.
When current is applied to the LED 14 from the patterns 16a and 16b through conductive members 11a and 11b, the LED emits light 17. Heat generated in the LED 14 is transmitted to the print substrate 16 through the conductive members 11a and 11b, so that the heat is efficiently radiated from the print substrate 16 if the substrate is made of a material having high heat conductivity.
However, the print substrate 16 is generally made of cheap material such as an epoxy resin. As described above, the epoxy resin has low heat conductivity. Therefore, the heat is not sufficiently transmitted to the print substrate, thereby rising the temperature of the LED, and reducing the quality thereof.
Here, if the print substrate is made with a metal having high heat conductivity, the heat radiation problem is resolved. However, the print substrate is not provided for mounting only the light emitting device, but for mounting other electrical parts such as CPU, memories and others.
For such other parts, the print substrate having high heat conductivity is not necessary. To the contrary, such as a print substrate made of metal causes the manufacturing cost of the electric device to increase.